Field of the Invention
The invention relates to a disk-shaped silicon sensor element for a pressure sensor. The sensor element has a disk upper It side, a disk underside, stress measuring elements for converting mechanical stresses into signals, and circuits disposed on the disk upper side. The disk underside serves during a pressure measurement for receiving a force leading to bending of the disk-shaped silicon sensor element. The invention also relates to a pressure sensor in which such a silicon element is used.
For measuring pressures in a range from 1 to about 200 bar, measuring sensors with silicon-based pressure sensors are predominantly used.
In a first type of construction of silicon sensor elements, a thin diaphragm is formed in a region of a silicon carrier material by use of an isotropic or anisotropic etching technique. The diaphragm, which is much thinner than the remaining silicon carrier material, is deformed under the effect of pressure. The deformation is converted into an electrical signal at the locations at which the greatest pressure-proportional stresses occur, with the aid of piezoresistive resistors disposed in the silicon in the region of the diaphragm. The disadvantage of the measuring sensors is that different diaphragm sizes and thicknesses have to be produced for different pressure ranges, in order to meet the requirements concerning sensitivity and rupture pressure. Therefore a separate set of masks for each pressure range is necessary for the photolithographic production and consequently leads to higher costs. What is more, the micromechanical structuring by an etching technique is an rile expensive process step. To obtain as many sensors as possible from a silicon wafer, the surface area of the sensors is minimized, resulting in very small and thin diaphragms. At the same time, the region of the diaphragm must not be disturbed with regard to its mechanical behavior. Therefore, apart from the piezoelectric resistors mentioned, the silicon carrier material must remain unused in the region of the diaphragm.
In the case of a further type of construction of measuring sensors, polysilicon diaphragms produced in a CMOS process are used as capacitive pressure sensors. The diaphragms, under the effect of pressure, lead to a change in a capacitance between the diaphragm and a carrier substrate, which is electrically measured. The additional process steps in the CMOS process make the production of the diaphragms complex and expensive. What is more, in the case of this technology, the diaphragm dimensions must also be adapted to the respective pressure ranges, that additionally makes the production process more expensive.
For higher pressures, from about 200 bar, thin film sensors based on steel or ceramic diaphragms are predominantly used. However, sensors based on this technology cannot be miniaturized unrestrictedly. This is so since the resistors require relatively large dimensions to register the diaphragm deformation. Because, the actual sensor element and the entire sensor electronics are disposed outside the pressure medium, the technique is of interest in particular in the case of pressure measurements in aggressive media. However, this requires a coating and structuring of the individual diaphragms, which greatly increases the expense of the pressure sensors.
It is accordingly an object of the invention to provide a disk-shaped silicon sensor element for a pressure sensor and a pressure sensor using such a silicon sensor element which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which can be used over a wide pressure range, can be produced on the basis of a low-cost process and offers the possibility of minimizing the dimensions of the pressure sensor.
With the foregoing and other objects in view there is provided, in accordance with the invention, a disk-shaped silicon sensor element for a pressure sensor. The disk-shaped silicon sensor element contains a body having a disk upper side and a disk underside. The body has a given thickness being substantially uniform over an entire extent of the body. Stress measuring elements are supported by the body, the stress measuring elements convert mechanical stresses into electrical signals. Circuits are disposed on the disk upper side. The disk underside serves during a pressure measurement for receiving a force leading to bending of the body.
The object is achieved over the prior art sensor element described at the beginning by the inventive silicon sensor element having essentially the same thickness over its entire extent. The invention in this case departs from the approach, adopted in the prior art for cost reduction, of reducing the size of the diaphragm to accomplish better area utilization. Rather, by increasing the thickness of the pressure-sensitive diaphragm, it takes the opposite approach and, by simultaneously increasing the size of the diaphragm surface area, in this way achieves additional usable surface area for the arrangement of switching and memory elements, so that, in spite of increasing the overall dimensions, there is an overall increase in the area utilization.
Thus, according to the invention, the silicon sensor element continues to be used for pressure measurement in the low-pressure range, but, on account of its essentially constant thickness over virtually its entire extent, the element assumes the function of the diaphragm of the prior-art sensors. Consequently, it is possible to dispense with the production of a separate diaphragm in a silicon body by an expensive etching technique. On the other hand, the production method allows sets of masks that are independent of pressure ranges and allows the amount of material used to be reduced. Moreover, the silicon sensor element according to the invention can be fabricated by simple ASIC production steps, since all that happens is that the silicon carrier is ground thin to produce its final form.
On account of the constant thickness of the silicon sensor element, the invention offers the advantageous development possibility of using the disk upper side entirely for the configuration of measuring elements and further electrical circuits, which was not possible in the case of known pressure sensors. As a result, the sensors according to the invention can fully utilize the expensive silicon carrier material and are therefore less expensive overall.
Different processes by which stress measuring elements convert the mechanical stresses occurring due to the deformation during a pressure measurement into mechanical, electrical or optical signals are possible. It is particularly expedient, for example for the further processing in a processor, to convert the mechanical stresses into electrical signals by stress measuring elements.
Similarly, different techniques known per se can be used for registering the stresses introduced into the silicon sensor element. In a development of the invention, stress measuring elements which have piezoelectric resistors are advantageously used.
Since only the disk underside is required for the introduction of the force producing the bending, the free disk upper side is available for the configuration of devices, for example for processing the signals obtained. According to an advantageous refinement of the invention, therefore, memory elements and/or electrical circuits for signal processing are formed on the disk upper side.
Among the factors determining the thickness of the silicon sensor element are the pressure to be measured and the component dimensions to be achieved. A favorable value for the essentially constant thickness of the silicon sensor lies in the range between 10 and 70 xcexcm.
In the case of a pressure sensor, a sensor carrier expediently serves as a holding and protecting device for the silicon sensor element. According to the invention, a pressure sensor in which a silicon sensor element is connected to the sensor carrier in a direct and pressure-tight manner is provided. In this way, by dispensing with additional sealing carriers or connecting parts, a compact configuration is achieved, reducing the use of parts and the external dimensions of the pressure sensor.
As is known, the sensor carrier and other components of the pressure sensor subjected to pressure consist of pressure resistant material. Suitable as such a material are materials that can be cast, machined or connected by soldering, such as for example ceramic or steel. According to an advantageous refinement of the invention, the sensor carrier is fabricated from ceramic or steel in particular in the region of the pressure-tight connection. In this way it is ensured that the pressure-related deformation occurs exclusively in the region of the silicon sensor element and the measurement is not falsified by pressure losses within the pressure sensor.
To produce a pressure-tight connection between the silicon sensor element and the sensor carrier, force-locking and form locking types of connection and hybrid forms of these are suitable. The pressure-tight connection is advantageously established by one of the following types of connection: adhesive bonding, anodic bonding, eutectic bonding, soldering or glass soldering.
The adaptation of the pressure measuring sensor to the pressure range to be measured can take place in the low pressure range by the choice of a thickness of the silicon sensor element. In order to reduce the material expenditure for pressure sensors that are used in higher pressure ranges, however, the disk underside of the silicon sensor element may be advantageously connected to a pressure diaphragm over its full surface area and with force locking engagement. In the production of such a pressure sensor, it is possible to omit a working step inasmuch as the sensor carrier already has a pressure diaphragm with which the underside of the disk of the silicon sensor is connected over its full surface area and with a force-locking engagement. Consequently, the silicon sensor element is at the same time separated and protected from the pressure-bearing medium, which is advantageous in particular in the case of aggressive media. This construction additionally offers the advantage that the adaptation of a pressure sensor to a specific pressure range does not have to take place by the dimensioning of the expensive and sensitive silicon sensor element, but can be adjusted more easily, and consequently at lower cost, by the thickness of the pressure diaphragm of the sensor carrier. In this way, pressure sensors for pressure ranges of different levels do not differ in the choice of the silicon sensor elements, which are expensive due to the material and electronic components with which they are fitted, but in the use of easily adaptable and relatively inexpensive sensor carriers. As a result, the silicon sensor element becomes a standard component.
On the other hand, for pressure sensors to be used in the low pressure range, it is possible to dispense with a separate pressure diaphragm. The silicon sensor element is dimensioned in such a way that it withstands the pressure on its own and is connected to the sensor carrier over part of its surface area, so that it itself serves as a pressure-loaded diaphragm. This comes close to achieving a same-parts concept for pressure sensors from the low to high pressure measuring range, since the pressure sensors to be used for different ranges differ by only a few, easily producible, and consequently low-cost, components, whereas the complex and cost-intensive silicon sensor element does not vary at all in thickness, or only slightly.
Pressure sensors are used to determine the difference between a pressure occurring and an ambient pressure. For such relative pressure measurements, the silicon sensor element is disposed in the pressure sensor intended for it in such a way that it separates the pressure-loaded region from the atmospheric pressure. On the other hand, it may be required Ps to determine an absolute value, independent of the ambient pressure. For the absolute pressure measurement, pressure sensors are configured in an extremely simple way, in that the silicon sensor element is disposed in such a way that it separates the pressure-loaded region from a reference pressure region.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a disk-shaped silicon sensor element for a pressure sensor and a pressure sensor using such a silicon sensor element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.